Rewiring the heated bed on the Monoprice Select Mini 3D printer

 

The Monoprice Select Mini is a very popular 3D printer. It’s inexpensive, small, and very hackable/fixable. For those reasons, it is a popular 3D printer among newbies and experienced printers alike. Having said that, it has one very significant flaw that affects pretty much 100% of units. The Mini comes with a heated bed which is a nice feature for an inexpensive printer. However, the wiring design is terrible and the movement of the bed while printing breaks the wires that control the bed temperature, usually within a few weeks. Mini owners are lumped into two camps on this – those who rewire the bed so that it will not break again, and those who just don’t care that the bed no longer heats. I was in camp 2, but eventually wanted to graduate to printing with ABS. It’s easy to print PLA on a 3D printer without a heated bed, but it’s much harder to get good ABS prints on a cold bed. So, I sent my printer back RMA to get it fixed up good as new, and then promptly rewired the bed when it came back to ensure the wiring would not break again.

You know your wiring is gone when your Mini thinks the bed temperature is 999 degrees, or when it flickers back and forth between some reasonable temperature and this 999 value

The reason why this is such an issue with the Mini is because it has a moving bed. Most 3D printers have a stationary bed, and the print head moves on the X, Y, and Z axes. The Mini’s head only moves on the Y and Z axes, and the bed itself moves to provide movement along the X axis. The wiring for the heated bed and the thermistor to control its temperature come up from the motherboard inside the chassis and up through the bottom of the bed platform. At the point where the wires come up through the bed platform, they are bent in a 90 degree angle. That angle causes the wires to break very quickly once the bed starts moving back and forth during normal printing operations.

To prevent this from happening, the wires should be redirected so that they do not have that 90 degree kink in them. The easiest way to this is to bring the wires out the side of the chassis instead of up through the top. That leaves your wires much less kinked, and they can be run into the back of the bed to attach to the heat platform, instead of being brought up underneath where the belts and gears are.

There are two main ways to to this – the drill way, or the non-drill way. While both methods change the way the wires are routed through the chassis, the drill method requires you to drill a hole in the back of the printer. The no-drill method requires…um…no drilling. I don’t see any advantage to drilling so I’ve chosen the non-drill method. Keep in mind that my printer was just returned, working, from Monoprice. Therefore, I do not want to print anything on it until I’ve rewired it. This was a bit of a problem because part of this mod requires me to print some new parts. To get around this, I removed the wires, printed my parts on a cold bed, then put the whole thing back together. Here’s the deets:

1. (No) Tools required

You will need a phillips screwdriver to remove the bed screws, and something to snip cable ties with (pliers, scissors). That’s really all you need. I opted to use shrink-wrap and tie up the wires nicely at the end, but that’s all optional. You literally just need a screwdriver and something snippy.

2. Detach the wires from the motherboard and pull them out to avoid damage while printing mod parts

Open the side of the chassis that has the USB and SD card slot. You’ll find the motherboard right there. It’s not really possible to get your fingers in there to identify or remove the heater and thermistor wires so you will probably have to take the bottom of the chassis off as well. Once you’ve done that, you should have fairly easy access to the wiring.

Unplug two things – the heater wires and the thermistor wire. On my machine, the heater wires are thicker and on the back left corner of the motherboard if you are looking at the printer face on. The thermistor wire is two slots over and has a yellow plug. It’s exceptionally easy to trace the wires from the bottom of the bed to the motherboard, but if you’re nervous, here’s a couple of shots of where they plug in.

 

 

Here’s another shot of the motherboard with the two plugs removed and running.

 

 

Next, pull the wires up through the top of the chassis. I was able to wiggle the wires through the hole in the chassis with the plugs attached. If you have to remove the plugs from the wires, you may have a challenge getting them back on. You may also need to snip the existing cable ties at this point because the wires you want to remove may be strapped to wires you don’t want to mess with.

 

 

There’s one more step before we can print the parts we need. If we print like this, the wires will still get crimped a bit under the bed platform, so we need to pull them up through that platform as well so they’re just connected directly to the bed heating pad. Remove the bed by undoing the levelling screws all the way. Be careful because those screws are spring loaded and can fly away into the dark corners of your room if you’re not careful.

You want to end up with this configuration.

 

 

Now, get ready to print the parts you need. Put the chassis back together but leave the wires where they are just sticking off the back of the bed. In theory, you can reattach the wires to the motherboard at this point by going around the side of the chassis, but I elected to just tape the disconnected wires to the back of the chassis and print my parts on a cold bed.

 

 

I found the levelling screws to be a little hard to get back in. Put each one together in this order, making a little spring sandwich between the head and foot caps. Try to keep that pinched together with your fingers and put it between the bed and the bed platform, and then put the screw through the bed, all the way through the spring sandwich, and into the screw hole in the bed platform. Screw it down as much as you think is needed to get the bed lower than the home Z height.

 

 

Spring sandwich:

 

 

Next, level the bed. You’re going to print some parts now and your bed level is going to be completely out of whack because you’ve totally remove the bed. Once the printer is back together and the bed level, proceed to the next part.

2a. Notes about printing on a cold bed

If you’ve never printed on a cold bed, it’s entirely possible with PLA. I use a hot end temperature of 220. I put glue stick on the bed prior to printing. For good measure, I also use a brim and once the brim has a little depth to it, I tack it in place with some dabs from a hot glue gun. That last step prevents curling and since we’re going to print some large flat pieces, we really want to avoid curling.

 

3. Print the mod parts

You will need a new side panel and a cable-tie mount to hold the wiring to the back of the bed platform. I used these three parts. There are three parts because the side panel has to be printed in two parts due to its size, plus the cable-tie mount.

Note that the pictures in this section are from the Thingiverse pages – they are not pictures of my printer or my mod.

This is the back half of the new side panel. There are two on this page, I chose the one with the slots because I don’t have a fan on my motherboard that would necessitate the need for the panel with the hole in it.

This is the front half of the new side panel. Note that this thing also has a back panel, but it just has a hole to run the wires through. I liked the look of the back panel in the previous thing better so I used it. They fit together nicely, so one of them is probably a remix.

 

 

The whole reason I am going through this is because the wires break as they move around. Therefore, I want to make sure that the wires will move as little as possible from now on. This little cable tie screw mount is just what I need to affix the wires to the back of the bed platform.

Once you’ve printed these parts, proceed to the next section.

4. Reassemble the printer with the mod parts

Success! A properly heated bed once again.

It’s important to do this in the right order. I am choosing to shrink wrap my wires so that is step one. I threaded the wires through the smallest shrink wrap that would fit the plugs (there’s another argument for removing the plugs) and shrunk it down with a hair dryer.

I then affixed the cable-tie mount to the back of the bed platform, and then zap strapped the wires to it. To get the wires in the best position I pushed the bed all the way to the back because that is the longest run the wires will have to make. If you zap strap them too close to the bed, you’ll pull them when the bed moves all the way back.

Next step is to remove the bottom of the chassis again in order to get enough room to  plug the thermistor and heater wires back in to their correct spots.

Finally, I affixed the new side panels and started it up. My bed now holds a nice 50 degree temperature and I don’t have to worry about the wires breaking in a few days ruining the whole thing for me again.